Scanning apparatus for biological microdensitometry

ABSTRACT

A scanning device that includes a pair of opaque members which rotate about a common axis and lie across the path of radiant energy where one member has a transparent area in the form of a straight line and the other member contains a transparent area in the form of a logarithmic spiral so that when the discs are rotated at different speeds they produce a scanning aperture of constant shape and configuration formed by the intersection of the transparent areas.

DJU""LILG Mundkur 1 June 20, 1972 54] SCANNING APPARATUS FOR 2,967,9071/1961 Stamps ....350/272 x BIOLOGICAL MICRODENSITOMETRY 2,989,8916/1961 Rockafellow. ....350/272 x Inventor: Balaji Mundkur Storm, Conn.3,167,605 1/1965 Heldenham ..350/272 [73] Assignee: University ofConnecticut, Storrs, Conn. Primary ExaminerRoy Lake [22] Filed: June 18,1968 Attorney-Bevendge & De Grand1 [21] App]. No.: 737,967 [57] ABSTRACTA scanning device that includes a pair of opaque members U-S- Q t rotateabout a common axis and across the path of [51] Int. Cl. ..H0l 5/16,G02f 1/30 radiant energy where one member has a transparem area in [58]Fleld ofSearch ..250/233, 236; 350/272, 279, the f of a i h line and theother member Contains a 350/273 275; 356/169 transparent area in theform of a logarithmic spiral so that when the discs are rotated atdifferent speeds they produce a [56] References Cited scanning apertureof constant shape and configuration formed UNITED STATES PATENTS I bythe intersection of the transparent areas.

1,842,759 1/1932 Malm ..350/272 X 9 Claims, 8 Drawing Figures SCANNINGAPPARATUS FOR BIOLOGICAL MICRODENSITOMETRY BACKGROUND This inventionrelates to a scanning device which employs a pair of opaque membersrotatable around a common axis, and having transparent apertures whichintersect to provide a moving scanning aperture which traverses an imagefield.

One field where this invention finds particular utility is in connectionwith biological microdensitometry where cells or tissues are analyzed byusing a light microscope. Commonly, the cells or tissues are subjectedto cytochemical methods for the selective localization of substances byreactions whose end products are colored. Then, the degree of extinctionof monochromatic wavelengths specifically absorbed by the coloredreaction products is measured to determine the concentration of theparticular substance.

Similarly, the concentration of any specific chemical component inuntreated specimens may be determined if the peak of absorbedwavelengths is known for that component. For example, the concentrationof deoxyribonucleic acid may be found by subjecting the specimen to awavelength of 2,650 A. in the ultraviolet region and measuring theamount of light transmitted. Similarly, wavelengths of 2,800'A. may beused to evaluate quantitatively tyrosine in the specimen.

In common practice, these microdensitometric procedures involve thepositioning of a single, small fixed aperture in the microscope imageplane, over the area of interest inthe cell, followed by time-consumingmeasuring operations. The amount of monochromatic light absorbed by thisregion is then calculated in accordance with criteria well-establishedby the Beer-Lambert Laws. The optical requirements, aside from thespecial circumstances imposed by the microscope and the nature of thespecimen, are in general no different in biology than in conventionalspectrophotometric work with fluids contained in a cuvette. However, thebiologist has to contend with a greater number of variables anddifiiculties, such as lenticular glare arising from the microscope,inconstancy of optical path lengths, and other factors. These oftentimestend to lessen accuracy in addition to imposing certain physicaldrawbacks.

The chief of these is what is termed distributional error, i.e., aninaccuracy arising from inhomogeneous spread of the light-absorbingsubstance within a given area surveyed by a photomultiplier or otherdevices which detects radiant energy. This difficulty is practicallyunavoidable in cytology and it must be neutralized by resort to workingsequentially with one or two absorbing wavelengths in addition to onenormally employed.

In working with ultraviolet wavelengths and in precise, rapid work atall times with visible wavelengths as well, the prior art has recognizedthat techniques for scanning the specimen are mandatory. Scanningtechniques involve the rapid movement of a small aperture, in arepetitive pattern of parallel lines, in the image plane of a lightmicroscope. Alternatively, the microscope stage holding the specimenitself is moved in this fashion while a single stationary aperture inthe image plane scans different parts of the moving image, at successiveintervals of time. Flying-spot" methods have been used in conjunctionwith complex electronic apparatus which utilizes the principles ofscanning used in television technolo- 8)- All existing scanning devicesin biology are characterized by one or more drawbacks. For example, whenusing electronic or mechanical scanning devices, the scanning rasterinvolves unscanned lines or dead spots" between the scan lines. Portionsof the microscopic specimen, especially very small parts such asmitochondria or granules, if they are small enough to lie wholly or inpart along these unscanned lines, make no contribution to the process ofestimating extinction values and may introduce statistical errors duringintegration of the separate, instantaneous absorbtion values recordedduring the movement of the scanning aperture. The errors increase inmagnitude in direct proportion to the inhomogeneity of distribution ofthe light-absorbing cellular component.

In some instruments, the scanning aperture is circular, a shapeconducive to introduction of errors. If a circular scanning aperture isused, those parts of the microscopic specimen passing across thediameter of the moving aperture make a greater contribution to theintegrated absorbance than those passing through the edge.

In an effort to avoid such errors, circular scanning apertures arefrequently overlapped as in the original Nipkow disc of earlytelevision. A 50 percent overlap is common; it is unnecessary inbiological microspectrophotometry and may introduce random error bynon-uniform repetitive sampling of the same area or areas. The errorincreases in proportion to the non-random distribution of absorbingcomponents within cells. In addition, physical limitations preclude theuse of large numbers of very small, closely and accurately spacedapertures if good resolution is desired.

In certain mechanical scanning instruments, the nature of theconstruction may introduce errors originating in varying shapes of theaperture. In the instrument described by Deeley in the Journal ofScientific Instrumentation, Vol. 32, pages 263-267, a series of separateapertures is produced in the image plane by the intersection of straightlines cut in a single metal disc and a separate rectangular plate movingin a definite relationship to each other. The successive readingsyielded by the apertures separately at successive intervals of time areelectronically integrated and presented as an average count in about 4.5seconds per cell or part of a cell. The scanning apertures varycontinuously in shape from a true square at the center of the scanningarea to rhombii of varying angles at the periphery of the zone ofscanning. As a consequence, the areas of the scanning apertures areinconstant and constitute a source of error in critical quantitativemicroscopical measurement of specimens in which the absorbing componentis very inhomogeneously distributed and characterized by wide variationsin absorbance of light.

According to a preferred embodiment of my invention, a pair of opaquediscs with transparent openings are rotated about a common axis atdifferent angular velocities. It is important to stress that theseangular velocities bear a definite relation to each other. Thetransparent openings in the two discs intersect to provide a scanningaperture of quadrilateral shape which moves to expose, sequentially,substantially all of the field being studied. One disc carries at leastone linear, very narrow and elongated transparent opening which movesacross the field continuously, at a uniform speed, only once during ascanning cycle. The other disc has at least one thin elongatedtransparent opening in the form of a logarithmic spiral which movesacross the field repeatedly during the scanning cycle. The resultingscanning aperture or zone through the two discs moves longitudinallyalong the straight slot during the movement of the straight slot acrossthe field at a constant uninterrupted velocity.

A logarithmic spiral is, by definition, a spiral all tangents of whichform a same angle with the radius intersecting at the point of tangency.Such spirals are also called logistic or equiangular spirals. With thisdefinition in mind, it will be understood that there will be an area ofconstant shape formed at the intersection of a logarithmic spiral sloton one disc and a straight line slot on another disc, provided that thediscs move about a common axis and each slot is of uniform width. If thestraight line slot is radially oriented and the slots are of equalwidth, a rhombus having a constant angle will be formed.

The principal and most pertinent aspect of this invention involves onedisc with mirror-image spiral slots, which are preferably but notessentially logarithmic spirals. During their movement, the mirror-imagespirals intersect sequentially with a straight line slot on anothermoving member. Rapid movement of the spiral slots over the movingstraight slot will produce a smooth continuous movement of the resultingscanning aperture or zone up along one spiral and down along the otherspiral to cover the scanned field during the motion of the straight lineslot across the scanned field.

This invention enables the present device to scan a field by means ofapertures of uniform size and shape, which renders it particularlysuitable for use with devices such as photomultiplier tubes andassociated circuitry which measure and integrate, during a scanningcycle, the intensity of radiant energy passing through a specimen in amicroscope.

There are numerous advantages which result from this invention. First,it permits the elimination of appreciable dead spots in the scannedfield which exist in systems which employ electronic scanning techniqueswith their inherent raster lines. Second, the presence of aquadrilateral scanning aperture makes it unnecessary to overlap thepaths of movement of the scanning aperture.Third, it permits theconstruction of apparatus wherein the resultant scanning aperture mayhave a constant area, to ensure accuracy when used in connection withquantitative sensing means such as intensity-time integrating devicesusing photomultiplier tubes. Fourth, the invention permits the scanningof a field in a pattern whereby substantially each portion of the fieldis exposed for an equal interval of time, preferably only once andwithout overlap. Fifth, the moving parts are few, light and rotate atrelatively low speeds, thus simplifying construction and avoiding a needfor excessive maintenance.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic perspective view ofthe apparatus of the invention;

FIG. 2a, 2b and 2c show a preferred combination of discs used in theinvention, with FIGS. 20 showing the individual discs of FIGS. 2a and 2bin superposed relation;

FIGS. 30, 3b and 3c show another form of the invention, with FIG. 30showing the individual discs of FIGS. 3a and 3b in superposed relation;and

FIG. 4 shows a disc with a single logarithmic spiral of the typepreferred for construction of the discs shown in FIGS. 2b and 3b.

DESCRIPTION OF PREFERRED EMBODIMENTS Two possible embodiments of thisinvention are illustrated herein; however, it is understood thatnumerous variations thereof may be devised within the spirit of thisinvention.

FIG. 1 illustrates diagrammatically the principal elements employed in apreferred embodiment. A light microscope 2 has means for supporting aspecimen 4 and a conventional optical system which may yield a degree ofmagnification of between I,OOOX to 2,000X. The radiant energy from themicroscope moves parallel to the path P to a device for sensing radiantenergy such as the end-on-photomultiplier tube 6. A catadioptriccondensor focuses the radiant energy onto the cathode of thephotomultiplier tube 6.

The path of the radiant energy from the microscope to thephotomultiplier tube 6 is interrupted by an iris diaphragm 35 which hasa variable opening at 36 and a pair of opaque discs 10 and 12 which arerotatable about a common axis A. The discs are provided with fiber gears14 and 16 which mesh with the timing belts l8 and or other suitablegearing. Motive power for the belts 18 and 20 is provided by the motors22 and 24 which have gears 26 and 28 mounted on their shafts.

Due to the precise nature of the work performed in such apparatus andthe inherent need for accurate speed control, the motors 22 and 24 arepreferably of the synchronous, hysteresis type. The gears 26, 28 and anyothers required if the timing belts l8 and 20 are eliminated are made ofstainless steel or brass.

The disc 10 is, for the most part, opaque to the type of radiant energysensed by the photomultiplier tube 6. It is, how ever, provided withnarrow, elongated portions 30 and 32 which are transparent to theparticular type of radiant energy. If the disc 10 is a thin metal disc,the transparent portions 30 and 32 may be slots accurately formed in thedisc.

The nature of the slots 30 and 32 is perhaps best illustrated in FIG. 2bwhere it will be seen that they are mirror image spirals. Preferably,they are logarithmic spirals which, by

definition, are spirals, all tangents of which form an equal angle witha radius drawn through the point of tangency.

The disc 12 has a transparent portion 34 which is a straight line slotwhich may be radially aligned with the axis of rotation A of the disc12. This disc 12 is rotated by the motor 24 through the gear 28, timingbelt 20 and gear 16. Its angular velocity is substantially less thanthat of the disc 10.

The manner in which the discs 10 and 12 and their transparent portionsoperate to scan a field may be seen in FIG. 2c which shows the relativepositions of the transparent portions of the superposed discs. The smallcircular area 36 is formed by an iris diaphragm which permits adjustmentin the size of the field to be scanned, which is referred to herein asthe scanned field.

During a scanning cycle, the straight line transparent portion 34 movesacross the scanned field 36 only once. The disc 10 with its spiral slotsis moved in a manner whereby at least one of the spiral slots will passacross the radial slot 34 as the center of the radial slot moves througha distance equal to its circumferential dimension which, in this case,is its width. For equal scanning of the field, one or more spiral slotsmust move entirely across the radial slot during the time interval thatthe center of the radial slot moves through a path equal to itscircumferential dimension. Of course, the mirror image spirals will, insequence, come into intersecting alignment with the radial slot 34 and,due to their rapid movement thereacross, will produce a scanningmovement of the quadrilateral scanning aperture formed at theirintersection. Each movement of the scanning aperture will be smooth,continuous and at a constant velocity. Since the spiral slots are mirrorimages of each other, one spiral slot will produce a left-to-righttraversing movement of the scanning aperture while the other spiral slotwill produce a reverse movement of the scanning aperture. Properselection of the relative angular velocities of the discs 10 and 12 iscapable of producing a scanning pattern in which the field is scanned toan equal extent without substantial overlap. In this connection, it isdesirable to locate the field 36 toward the periphery of the disc and tominimize its area so that substantial errors will not result from theslight divergency between the various positions of the radial slot 34.

As mentioned previously, it is desirable to have the spiral slots 30 and32 in the form of logarithmic spirals. The advantage of this type ofspiral is that its tangents will always form a same angle with radiallines drawn to the point of tangency; and, therefore, the scanningaperture produced by such a spiral and a straight line on aconcentrically rotatable disc will have a uniform shape at all times.This principle is best illustrated in FIG. 4 which shows a singlelogarithmic spiral 38 which may be on one disc and which may intersect astraight line transparent portion 40 on another disc.

It will be observed that the angles 0, and 6 which are formed by theradial lines r, and r and the tangent lines I, and 2 are equal.Accordingly, the scanning aperture 42 which results at the intersectionof the transparent portions 38 and 40 will always have the same shape.If the width of the transparent slots 38 and 40 are equal and if theslot 40 is radially oriented, the scanning aperture 42 will always be arhombus having a constant angle and a constant area.

The significance of providing a quadrilateral scanning aperture with aconstant area will be appreciated when one considers the nature of thequantitative measuring system employed in apparatus of this type withthe photomultiplier tube. In order to measure the amount of a particulartype of radiant energy arriving at the photomultiplier tube during thescanning of a field, a standard electronic circuit is used to integratethe intensity of the light arriving on the photomultiplier tube 6 andthe time. If, when using such apparatus, the area of the scanningaperture 42 varies appreciably, errors would be introduced. For example,if the scanning aperture became larger during an interval of time whenscanning a less dense area of the specimen and small when scanning avery dense portion of the specimen, a misleadingly low reading would beobtained. Similar errors would arise if the scanning aperture movedacross the field at an irregular velocity. This is avoided by thepresent apparatus.

The arrangement shown in FIGS. 3a-c employs many of the principles alsofound in the apparatus of FIG. 2. In FIG. 3a, a single disc 44 ismovable only in one direction and has a series of spaced apart radiatingslots 46. The superposed disc 48 shown in FIG. 3b has a series of threelogarithmic spirals 50 which inherently produce the advantages discussedin connection with FIG. 4. These logarithmic spirals are geometricallyidentical to each other in that they conform to the same mathematicalformula and they are similarly oriented on the disc. When the discs areplaced together across the path of radiant energy, the alignment oftheir openings will be as represented in FIG. 3c, where 52 designatesthe scanned field. In this particular arrangement, only one of theradial slots 46 moves across the field 52 during each scanning cycle,while the logarithmic spirals 50 move rapidly thereacross to produce asweeping action in a single direction. The velocity of the disc 48 orthe spacing of the spirals 50 thereon is such that the radial disc wouldthen move through a distance equal to one half its circumferentialdimension both during and between successive sweeps of any of the spiralportions 50. The angular velocity of the disc 48 may be increased andwill provide equal exposure of the scanned field if a spiral orgeometrically identical spirals pass across the radial slot 44 any wholenumber of times and the slot moves through one-half its circumferentialdimension.

In one installation constructed in accordance with the diagrammaticillustration in FIG. 1 the discs and 12 were electroformed from thinbi-metallic sheets of copper and nickel having a thickness ofapproximately 1.5 to 2.0 mm. The width of the individual slots 30, 32and 34 is 0.2 mm. The area of the scanned field is controlled by an irisdiaphragm having a maximum diameter of 22 mm which encompasses only 20of the movement of the radial slot 34. The disc 10 was-rotated at anangular velocity of 360 rpm, while the disc 12 was rotated only throughthe 20 are at an angular velocity of 0.5 rpm. The drive mechanism forthe disc 12 permitted reversal of its rotation to return the slot 34 toits original starting position. This may be done by various mechanicalmeans including limiting stops and microswitches which are controlledautomatically through external equipment. The excursion of the slot 34is initiated by a pulse from a Hewlett-Packard Preset Counter Model52141. simultaneously with the opening of the counter gate formeasurements. The excursion of the slot 34 is terminated by amicroswitch which simultaneously cuts off electric power to the motor 24and actuates the preset counter to deliver a digital reading(totalizing, ratio, frequency, preset, etc.) of the signal relayedthrough the photomultiplier tube 6. A clutch and lever arrangement maybe used for returning the slot 34 to its starting position eithermanually or electrically.

In one arrangement, the logarithmic slots 32 and 30 were arranged sothat an angle corresponding to the angle 6, and 0 in FIG. 4 was 8l.75,thus producing a scanning aperture in the form of a rhombus having anangle of 8 l.75. The formula for such slots 30 and 32 is as follows:

where r is the radius vector 0 is the angle in radians The limits ofsuch a logarithmic spiral are between 20 and 165 of the disc and itsdistance from the axis A would range from 56 to 80 mm.

In precision work, it is naturally important that the transparentportions or slots in the disc 10 and 12 must be very accurately locatedand dimensioned. One manner of manufacturing such a disc is initially toproduce patterns on photo graphic emulsions or on Mylar sheets by amechanical drafting machine capable of transcribing patterns from datareceived from computer-programmed tapes or cards. These patterns areappreciably larger than the final dimensions desired for the disc andtherefore they are reduced in size before being applied to the metalwhich eventually is to become the disc.

Transfer of the patterns to the metal may be accomplished by a suitablewell-known photoresist technique such as those commonly employed in themanufacture of printed circuits and solid state electrical circuitry.

For situations where accuracy is not of paramount importance, the discsmay be made of quartz, fused, silica, glass or plastic which are coatedwith an opaque material which has scribed transparent areas to providethe transparent spiral and radius portions of the disc.

A suitable photomultiplier tube 6 is produced by the Electrical andMusical Industries Limited of England under EMI Model 955 which hasextended spectral sensitivity ranging from the low ultraviolet to theinfrared ranges. The housing for this tube is exchangable, by means ofadapters, with housing for other photomultiplier tubes such as the RCAIP21 ,IP 28 or 931A for less stringent applications. The signal from thephotomultiplier is amplified and its strength assessed by agalvanometer, a recording photometer or an oscilloscope.

Rather than use the photomultiplier tube and its associated circuitry,the scanning device may also be used in connection with means forvisually reproducing the scanned field. Such visual means may include aphotographic camera or conventional Orthicon or Vidicon tubes used intelevision applications which, of course, would be used in conjunctionwith suitable devices for reproducing the received image. Thesetechniques may necessitate movement of the discs at greater velocitiesthan previously described which would require use of the disc shown inFIG. 3a with any other suitable disc having the spiral transparentportions.

Within the spirit of the invention, numerous modifications may be madeto the disclosed apparatus. A single motor may drive both discs throughsuitable gearing. The concept of the mirror image spirals may, forexample, be used in conjunction with nonlogarithmic spirals. The radialslots may be inclined from their radial orientation to any positionwhere they will sweep across the field once during a scanning cycle. Oneorientation of the slots 46 would result in a square scanning aperture.

Furthermore, the width of the slots may vary along their length. In thisconnection, it may be found desirable to increase the width of the sloton the slowly moving disc toward its periphery in order to avoid anysmall dead sports or overlaps resulting from the slight divergencebetween the various positions of the slot. If this were done, the spiralslot may be reversely shaped with its narrower portion being toward theperiphery of the disc so that the resultant scanning aperture would atall times maintain a uniform area.

The velocities of the disc may also be changed as desired by theoperator. It is important that the entire field be scanned to an equalextent, but this may be accomplished by scanning it two, three or moretimes during an incremental movement of the slot on the slowly movingdisc.

Other modifications will naturally occur to those working in the fieldand are intended to be encompassed within the spirit of this inventionand by claims which follow.

What is claimed is:

1. Scanning apparatus for controlling the passage of radiant energy to aradiant energy sensing device, said apparatus having two membersrotatable about a common axis and lying across a path of radiant energyto the sensing device, each of said members having opaque areas forinterrupting the passage of radiant energy to the sensing device andnarrow and elongated transparent areas which permit the passage ofradiant energy to the sensing device, driving means for moving saidmembers at different velocities relatively across the path of radiantenergy leading to the sensing device with their transparent areas beingin intersecting alignment to form a moving transparent scanning zonewhich moves in plural passes across different portions of a scannedfield, said scanning zone being smaller in all dimensions than thefield, one of said members having a pair of said transparent areas inthe form of spirals which are mirror images of each other.

2. Scanning apparatus according to claim 1 wherein said spirals arelogarithmic spirals.

3. Scanning apparatus according to claim 1 in combination with a lightmicroscope, said sensing device lying in the path of light emerging fromthe microscope.

4. Scanning apparatus for controlling the passage of radiant energy to aradiant energy sensing device, said apparatus having two memberscoaxially mounted for independent rotation lying across a path ofradiant energy to the sensing device, each of said members having opaqueareas for interrupting the passage of radiant energy to the sensingdevice and narrow elongated transparent areas which permit the passageof radiant energy to the sensing device, the transparent area on one ofthe members being a radial linear slot located to move across the entirefield, and the transparent area on the other said member being in theform of a logarithmic spiral, driving means for moving said members atdifferent velocities relatively across the path of radiant energyleading to the sensing device with their transparent areas being inintersecting alignment to form a moving transparent scanning zone whichmoves in plural passes across different portions of a scanned field,said scanning zone being smaller in all dimensions than the field,whereby the transparent areas intersect at an angle which remainsconstant throughout their movement across the field and provide a movingtransparent scanning zone formed by the intersection of the transparentareas which maintains a constant area throughout its movement across thescanned field.

5. A mechanical scanner according to claim 4 wherein the driving meansmoves the linear slot through a distance equal to its circumferentialdimension as one of said logarithmic spirals is passed completelythereacross at least one whole time.

6. A mechanical scanner according to claim 4 wherein the other saidmember has a pair of transparent portions in the form of logarithmicspirals which are mirror images of each other.

7. A mechanical scanner according to claim 4 wherein the transparentareas have an equal width, thereby producing a transparent scanning zonein the form of an equilateral quadrilateral.

8. A mechanical scanner according to claim 4 in combination with a lightmicroscope, said sensing device lying in the path of light emerging fromthe microscope.

9. A mechanical scanner according to claim 4 wherein the moving scanningzone maintains a constant quadrilateral shape throughout its movementacross the scanned field.

1. Scanning apparatus for controlling the passage of radiant energy to aradiant energy sensing device, said apparatus having two membersrotatable about a common axis and lying across a path of radiant energyto the sensing device, each of said members having opaque areas forinterrupting the passage of radiant energy to the sensing device andnarrow and elongated transparent areas which permit the passage ofradiant energy to the sensing device, driving means for moving saidmembers at different velocities relatively across the path of radiantenergy leading to the sensing device with their transparent areas beingin intersecting alignment to form a moving transparent scanning zonewhich moves in plural passes across different portions of a scannedfield, said scanning zone being smaller in all dimensions than thefield, one of said members having a pair of said transparent areas inthe form of spirals which are mirror images of each other.
 2. Scanningapparatus according to claim 1 wherein said spirals are logarithmicspirals.
 3. Scanning apparatus according to claim 1 in combination witha light microscope, said sensing device lying in the path of lightemerging from the microscope.
 4. Scanning apparatus for controlling thepassage of radiant energy to a radiant energy sensing device, saidapparatus having two members coaxially mounted for independent rotationlying across a path of radiant energy to the sensing device, each ofsaid members having opaque areas for interrupting the passage of radiantenergy to the sensing device and narrow elongated transparent areaswhich permit the passage of radiant energy to the sensing device, thetransparent area on one of the members being a radial linear slotlocated to move across the entire field, and the transparent area on theother said member being in the form of a logarithmic spiral, drivingmeans for moving said members at different velocities relatively acrossthe path of radiant energy leading to the sensing device with theirtransparent areas being in intersecting alignment to form a movingtransparent scanning zone which moves in plural passes across differentportions of a scanned field, said scanning zone being smaller in alldimensions than the field, whereby the transParent areas intersect at anangle which remains constant throughout their movement across the fieldand provide a moving transparent scanning zone formed by theintersection of the transparent areas which maintains a constant areathroughout its movement across the scanned field.
 5. A mechanicalscanner according to claim 4 wherein the driving means moves the linearslot through a distance equal to its circumferential dimension as one ofsaid logarithmic spirals is passed completely thereacross at least onewhole time.
 6. A mechanical scanner according to claim 4 wherein theother said member has a pair of transparent portions in the form oflogarithmic spirals which are mirror images of each other.
 7. Amechanical scanner according to claim 4 wherein the transparent areashave an equal width, thereby producing a transparent scanning zone inthe form of an equilateral quadrilateral.
 8. A mechanical scanneraccording to claim 4 in combination with a light microscope, saidsensing device lying in the path of light emerging from the microscope.9. A mechanical scanner according to claim 4 wherein the moving scanningzone maintains a constant quadrilateral shape throughout its movementacross the scanned field.